1. Field of the Invention
The present invention relates to a fuel cut-off and fuel-supply recovery control system for an internal combustion engine coupled to an automatic power transmission with a so-called lock-up torque converter, and specifically to technologies for optimally controlling the timing of beginning of a so-called fuel cut-off control and the timing of beginning of a so-called fuel recovery control in an automotive engine coupled to an automatic transmission employing a lock-up torque converter.
2. Description of the Prior Art
Recently, there have been proposed and developed various fuel cut-off systems which are designed to reduce exhaust emissions and fuel consumption by cutting fuel supply to the engine cylinder or cylinders through the fuel cut-off control. On more later-model cars with a fuel cut-off system, the fuel cut-off control is frequently performed under certain required engine/vehicle operating conditions, such as during vehicle deceleration, during coasting or down-hill operation, to obtain deceleration fuel cut-off, as well as engine speed limitation when the maximum allowable engine speed is reached. For example, when the vehicle is decelerated at engine speeds above a first predetermined reference engine speed, generally called as a "fuel-cut engine speed", the fuel cut-off operation continues until the engine speed drops to the first predetermined reference engine speed. In the conventional fuel cut-off system, a second predetermined reference engine speed is also used to restart or recommence fuel supply to the automotive engine when the engine speed excessively drops down to the second predetermined reference engine speed, to prevent engine stalling which may take place during deceleration fuel cut-off operation. The second predetermined reference engine speed is determined as the minimum possible engine speed capable of executing fuel-cut operation or as an engine-speed lower limit below which there is a risk of engine stall even when the fuel supply is recommenced. The second reference engine speed is generally called as a "fuel-cut recovery engine speed" or simply as a "fuel-recovery engine speed". One such fuel cut-off system has been disclosed in Japanese Patent Provisional Publication No. 58-57048, assigned to the assignee of the present invention. Japanese Patent Provisional Publication No. 58-57048 teaches the provision of a predetermined speed difference (often called an "engine-speed hysteresis") between the fuel-cut engine speed and the fuel-recovery engine speed. Usually, the fuel-cut engine speed is set at a higher level than the fuel-recovery engine speed to prevent undesired hunting. In the prior art system as disclosed in Japanese Patent Provisional Publication No. 58-57048, the aforementioned engine-speed hysteresis is fixed to a predetermined value. According to the deceleration fuel cut-off and fuel-supply recovery control of the system disclosed in Japanese Patent Provisional Publication No. 58-57048, basically, the fuel cut-off control will be brought into operation when the detected engine speed exceeds the predetermined fuel-cut engine speed, whereas the fuel-supply recovery control will be brought into operation when the detected engine speed drops below the predetermined fuel-recovery engine speed. Additionally, the prior art fuel cut-off system disclosed in Japanese Patent Provisional Publication No. 58-57048 is in combination with an automatic transmission control system, for forcibly down-shifting the automatic transmission in response to a braking signal indicating a braking operation and for beginning the fuel-cut again at the predetermined fuel-recovery engine speed lower than the predetermined fuel-cut engine speed when the brakes are applied during coasting, thus increasing the fuel cut-off time duration owing to a rise in engine speed caused by the down-shift. In recent years, many cars are equipped with automatic transmissions with so-called lock-up torque converters which act to mechanically couple the engine crankshaft to the transmission output shaft. As is well known, when a lock-up device incorporated in the lock-up torque converter assumes its converter state (corresponding to a lock-up clutch release position), the engine crankshaft and the transmission output shaft are coupled via fluid in the torque converter in normal operation. On the contrary, when the lock-up device assumes its locked-up state (corresponding to a lock-up clutch engagement position), the engine crankshaft and the transmission output shaft are directly coupled to each other by a mechanical connection (via the lock-up clutch engaged), thus disabling or locking up the torque converter. The converter state will be hereinafter referred to as a "lock-up OFF state", whereas the locked-up state will be hereinafter referred to as a "lock-up ON state". As is generally known, the lock-up clutch includes an apply chamber and a release chamber. The lock-up clutch is generally controllable by the pressure difference between the apply pressure in the apply chamber and the release pressure in the release chamber to operate at either one of at least an open converter zone and a full lock-up zone. In modern automotive vehicles employing automatic transmissions with lock-up torque converters, the lock-up clutch tends to be operated in the lock-up ON state during vehicle coasting in which torque-increase and torque-fluctuation absorbing functions are unnecessary. The lock-up ON state occurring during coasting will be hereinafter referred to as a "coasting lock-up state". In general, the fuel cut-off system operates to cut the fuel supply to the engine during the coasting lock-up state, thus saving fuel and emissions. Such simultaneous execution of both the lock-up operation and the fuel cut-off operation are very effective to reduce fuel consumption, while preventing engine stall.